From that RFC:

0.0.0.0/8 - Addresses in this block refer to source hosts on "this"
network.  Address 0.0.0.0/32 may be used as a source address for this
host on this network; other addresses within 0.0.0.0/8 may be used to
refer to specified hosts on this network ([RFC1122], Section
3.2.1.3).

(note that it only says "source address")

which was based on RFC 1122, which states:

We now summarize the important special cases for Class A, B,
and C IP addresses, using the following notation for an IP
address:

    { <Network-number>, <Host-number> }

or
    { <Network-number>, <Subnet-number>, <Host-number> }

...

(a)  { 0, 0 }

This host on this network.  MUST NOT be sent, except as
a source address as part of an initialization procedure
by which the host learns its own IP address.

See also Section 3.3.6 for a non-standard use of {0,0}.

(section 3.3.6 just talks about it being a legacy IP for broadcasts - I don't think that even works any more)

it makes connecting to localhost as easy as http://0:8080/ (for port 8080, but omit for port 80).

The thing is that it's not supposed to work, so it's essentially relying on undefined behaviour. Typing [::1]:8080 is nearly as easy.

skimming through these PRs, at least for WebKit, I don't see tests for shorthand IPs like 0 (and no Apple device to test with). What are the chances they missed those..?

I haven't seen the PRs, but IP comparison should really be using the binary form of the IPv4 address (a 32-bit number), not the human-friendly form.

notably

Windows is not impacted by this issue.

quoting the main, critical part:

1. Under public domain (.com), the browser sent the request to 0.0.0.0.
2. The dummy server is listening on 127.0.0.1 (only on the loopback interface, not on all network interfaces).
3. The server on localhost receives the request, processes it, and sends the response.
4. The browser blocks the response content from propagating to Javascript due to CORS.

This means public websites can access any open port on your host, without the ability to see the response.

I didn't realize DEFCON was this weekend already, but this is a solid point 😂